An automotive vehicle includes a seat belt system to restrain a vehicle occupant in a seat during vehicle collision so as to protect the occupant from a second collision.
When such a seat belt system is used to restrain an occupant, a seat belt must be fastened so as not to separate the seat belt from the occupant, or leave a space between the seat belt and the occupant. Otherwise, a decrease in the restraint effect occurs. This is due to the fact that during collision the occupant is forced, due to inertia, to move by a distance corresponding to the space between the seat belt and the occupant.
In order to prevent such a decrease in the restraint effect, there has been employed a seat belt retractor with a pretensioner system constructed as shown in FIG. 4.
As shown in FIG. 4, such a seat belt retractor with a pretensioner system comprises a reel shaft 3 around which a seat belt 5 is wound, the seat belt 5 being retractable into and extractable from the reel shaft 3, a pulley 4 fixedly mounted to one end of the reel shaft 3, a wire rope 2 composed of a plurality of metal wires and wound, with a predetermined space, around the pulley 4 a plurality of times, and a pull device 1 for pulling the wire rope 2.
During vehicle collision, such a conventional seat belt retractor with pretensioner system operates as follows.
Means (not shown) is provided to sense sudden acceleration of a vehicle and generate an electrical signal. The pull device 1 is responsive to this signal so as to rapidly pull the wire rope 2 in the direction of the arrow A.
The wire rope 2 is then wound around the pulley 4. The friction between the wire rope 2 and the pulley 4 causes the pulley 4 to rapidly rotate in the direction of the arrow B.
As the pulley 4 is fixed to the reel shaft 3, the rotation of the pulley 4 causes retraction of the seat belt 5 into the reel shaft 3. This eliminates the space between the occupant and the seat belt, or loosening of the seat belt.
It is considered that the seat belt retractor with the pretensioner system thus constructed is capable of preventing such a decrease in the restraint effect since the seat belt 5 is rapidly brought into close contact with the occupant during collision.
However, in the seat belt retractor with the pretensioner system thus constructed, an unexpected amount of slippage occurs between the wire rope 2 and the pulley 4 when the wire rope 2 is rapidly pulled. It is, therefore, difficult to adequately control the length of seat belt 5 taken up or pulled back.
If the amount of slippage between the wire rope 2 and the pulley 4 is greater than an expected value, the seat belt 5 is not sufficiently brought into close contact with the occupant. On the contrary, if the amount of slippage is less than an expected value, there is a problem that the seat belt 5 too tightly extends across the occupant.
Also, in the prior art seat belt retractor with the pretensioner system, the reel shaft 3, rather than the seat belt 5 per se, is rotated to eventually retract the seat belt 5.
As such, even if the seat belt 5 is appropriately brought into close contact with the occupant by rotating the reel shaft 3 to retract the seat belt 5, the occupant is forced to move forward due to inertia to cause the seat belt 5 to be tightly wound around the reel shaft 3. This allows the occupant to move forward, thus failing to effectively restrain the occupant.